X-ray microscopy is a technique that offers unique imaging through its combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and ease of image interpretation. In the past, high resolution X-ray microscopy has been restricted to a few synchrotron radiation laboratories around the world. The emergence of laboratory source-based x-ray microscopes holds the opportunity to make this imaging modality much more widely available. Such laboratory-source x-ray microscopes, however, rely on the availability of high brightness x-ray sources for high performance.
Resolution and throughput are two important parameters defining the performance of a microscope. The former defines smallest features that can be imaged, while the later defines how fast useful information can be obtained. For a full field x-ray microscope, the exposure time T is inversely proportional to the flux F incident on the object:F=ηBcL2Δθ2,  (Ex. 1)
where Bc, L, and Δθ are the beam brightness, the field of view, and the divergence of the illumination beam at the object, respectively; η the efficiency of the focusing optics. Expression (1) shows that for a given field of view L, divergence Δθ, focusing efficiency η, F is proportional to the source brightness Bc. Therefore, a brighter x-ray source means shorter exposure time and thus higher throughput.
A brighter x-ray source also permits higher resolution for a given exposure time. The dependence of exposure time T on resolution δ is approximately given byT=a/δ4,  (Ex. 2)
where “a” is a parameter independent of resolution and related to image contrast and the imaging system efficiency.
Expressions (1) and (2) show that for a given exposure time and imaging objective, the resolution can be improved by a factor of B1/4 for a brighter source. This factor equals to 1.56 for a 6× brighter source.
The most widely deployed laboratory sources generate x-rays by bombarding energetic electrons into a target (anode), similar to how Roentgen first generated x-rays in his laboratory. The resulting x-rays consist of narrow-band characteristic x-rays resulting from ionization and de-excitation of core electrons and continuous Bremsstrahlung (braking) radiation resulting from the deceleration of the energetic electrons. Except for commercial x-ray applications requiring sources with a high intensity as the main requirement such as medical radiography and medical CT, or luggage scanners, a significant number of applications such as x-ray microscopy, protein crystallography, and small angle scattering, requires a source with high brightness for the characteristic x-rays.
The key limiting factor for increasing brightness of this type of source is the melting of the anode target. Two well-established approaches have been developed to overcome this limitation and are used in current high brightness laboratory x-rays sources. The first method facilitates thermal dissipation by using a fast rotating anode target to distribute the heat flux over a large area to prevent the target from melting. X-ray sources based on this method constitute the most powerful x-ray sources widely used in a home-lab environment. The second method uses a micro-sized electron spot (microfocus source) to reduce the thermal path to produce a large thermal gradient for better thermal dissipation.
Several other approaches have been explored in recent years to produce high brightness laboratory x-ray sources. One method involves innovations based on various forms of accelerator-based technologies and two miniature synchrotron sources have been demonstrated recently. The accelerator and miniature synchrotron sources are currently expensive. Another method uses a high power laser beam focused to a small spot on a target to produce high temperature plasmas that emit high brightness x-rays. However, this method is limited to soft x-rays and not well suited for multi-kiloelectron Volts (KeV) x-rays that are desired for most for imaging.